The present invention relates to a fieldbus network and, more particularly, to a current limiter to protect a fieldbus network from an electrical short in a spur cable or a device attached to a spur cable.
In a typical industrial plant application, sensors measure position, motion, pressure, temperature, flow, and other parameters related to the operation of process machinery and activities. Actuators, such as valves and motor controllers, control the operation of the machinery and process activities. The sensors and actuators are remotely located from the human and computerized controllers which gather information from the sensors and direct operation of the actuators. A communication network links the controllers with the sensors and actuators located in the field.
Heretofore, communication between controllers, remote sensors, and actuators in industrial applications has been by means of analog signaling. The prevailing standard for analog networking of field devices and the control room in industrial applications has been the Instrument Society of America standard, ISA S50.1. This ISA standard provides for a two-wire connection between the controller and each field device. One wire of the system carries the analog signal between the remote device and the controller. The analog signal may be converted to a digital signal useful to a computerized controller. The second wire of the circuit supplies DC power for operation of the remote sensor or actuator. Both wires provide return paths for data and power respectively
Communication utilizing digital signaling reduces the susceptibility of the communication system to noise and provides a capability for conveying a wide range of information over the communication network. Digital communication also permits several different devices to communicate over a single pair of wires. Remote devices used in connection with a digital communication system typically incorporate local “intelligence.” This permits sensors and actuators to perform diagnostic, control, and maintenance functions locally. Further, the local intelligence permits the devices to communicate directly with each other and perform some functions without the necessity of involving a central control facility, thus promoting the development of distributed control systems.
Fieldbus is a generic term used to describe a digital, bidirectional, multidrop, serial communication network for connecting isolated field devices, such as controllers, actuators and sensors, in industrial applications. One such fieldbus called Foundation Fieldbus or “FF” is defined by the Instrument Society of America standard, ISA SP50.02. This system utilizes a two-wire bus to provide simultaneous digital communication and DC power to remotely located devices.
A current limiter is disclosed in U.S. Pat. No. 6,366,437 which is assigned to the same assignee, the contents of which are incorporated herein by reference. The protection circuit of the '437 patent minimizes the impact of short circuits on a fieldbus network. It does this by limiting the current that is supplied from the home run cable to the spur cables and connected devices. Any short or overcurrent condition on the spur cable results in the failure of any devices on that spur but the rest of the spurs, with their associated devices and the network, continue to operate normally as long as there is sufficient current for the trip point on the protection circuit.
Another device network called AS-interface, or ASi, is similar to FF but ASi devices use substantially more current. To raise the current trip point limit on a circuit such as disclosed in U.S. Pat. No. 6,366,437, could cause the circuitry to overheat and self-destruct during a sustained short circuit because of the amount of power available on the network. Moreover, on some FF networks, especially those used for intrinsically safe installations, the available power is limited in order to prevent a spark from causing an explosion in the hazardous area. In such a case, the difference between the power the device on the spur uses and the protection circuit trip point current is more than is available on the segment. A short on the spur in this situation would not prevent failure of the entire segment.